This invention relates to variable attenuators for microwave signals, and particularly to such attenuators or amplitude modulators that can be digitally programmed over a broad range of radio frequencies.
An attenuator that may be programmed for such use is a switched resistive-pad attenuator in which resistive pads of different values, or combinations of values, of attenuation are switched in and out of operation as desired. Such attenuators are suitable for switching large steps of attenuation at high speed. However, there are situations where the programmed steps of attenuation are very small (e.g., up to several db) and where accuracy requires coming within a small percentage of final value (e.g., 0.1 or 0.2 db). Then the attenuation of the pin diode itself is a significant part of the attenuation step, and the switching speed of the pin diode is also a significant factor of the overall switching speed. Such attenuators, in the latter situations, particularly in frequency ranges above 8 GHz, tend to be too slow in switching speed; in addition they have linearity and flatness problems, and they have high insertion loss.
Another form of attenuator is one employing variable reactors; an example is an attenuator (such as that of U.S. Pat. No. 3,346,823) based on the use of an analog phase shifter. Even where such a phase-controlled attenuator employs the phase shifter of U.S. Pat. No. 4,288,763, such a device tends to be limited in its attenuation range in a single attenuator (using two or three phase-shifter stages). For a large operating range of attenuation, cascaded identical phase-controlled attenuators would be required, but in such a multi-attenuator configuration, the deviations from flatness in each attenuator are cumulatively excessive. Moreover, such a configuration would have high insertion loss also.